In switching power supplies, an oscillator is usually formed by a current source 11, a capacitor C1, a switch S1, a comparator COM1 and a one-shot circuit, connected as shown in FIG. 1. The capacitor C1 is basically charged by the current source 11. The comparator COM1 compares the voltage Vc1 across the capacitor C1 with a reference voltage Vref. Once the voltage Vc1 increases to reach the reference voltage Vref, a pulse will be generated by the one-shot circuit to discharge the capacitor C1 through the switch S1.
Theoretically, the oscillating frequency f of the oscillator is determined by the reference voltage Vref, the current source 11 and the capacitor C1. Nevertheless, owing to an intrinsic delay Td of the comparator COM1, the switch S1 could not be turned on right after the voltage Vc1 hits the reference voltage Vref, as can be seen from FIG. 2. The frequency f could be expressed as:
                    f        =                              1                                          T                saw                            +                              T                op                                              =                      1                                                                                C                    1                                    ⋆                                      V                    ref                                                                    I                  1                                            +                              T                d                            +                              T                op                                                                        (        1        )            Wherein Tsaw indicates the charge time of the capacitor C1, and Top is representative of the width of the pulse generated by the one-shot circuit.
The inherent delay Td is generally affected by temperature, parasitic inductance and etc. And because of this delay, the oscillating frequency f could hardly match its theoretical value, especially for high frequency oscillators with frequency higher than 2.2 MHz.